In the prior art, as shown in Japanese patent publication number 0128079, the technology includes right and left travel crawlers supported by drive sprockets and tension rollers installed to the right and left track frames through track rollers. The right and left track frames are installed on this machine through crawler elevation arms in a manner where the track frames are movable up and down by correcting the lateral tilt. Said prior art can prevent tension from varying by minimizing changes in the ground contact area of the right and left travel crawlers because, when this machine is elevated, the track frames are lowered in order to come close to the drive sprocket side. There are, however, problems where a shift in the center of gravity of the machine body toward the rear side of the lateral ground contact width of the travel crawler results in a tilting of the machine body backward, followed by the elevation of the moving section in front of the machine body, thus leading to a change in the reaping height, as well as problems where the backward shift in the center of gravity of the machine body increases the ground contact pressure on the rear side of the travel crawler and the backward travel facilitates the rear sides of the travel crawlers to penetrate into the earth, thus degrading the turning performance at the bare end area of a wet rice field.